1. Field of the Invention
The present invention concerns a superconductive electrical transmission line with two functions: transportation of current and current limitation.
2. Description of the Prior Art
The invention finds one specific but non-limiting application in an electric power production plant comprising a medium-voltage (for example 20 kV) generator connected by a transmission line a few hundreds of meters long to a medium-voltage/high-voltage (for example 20 kV/400 kV) transformer station. The transmission line carries a very high current (for example on the order of 20 kA); large cross-section (500 mm diameter) conductors are usually employed, cooled by a fluid (usually air) flowing in a large-diameter (typically one meter) metal pipe. The losses in a transmission line of this kind are on the order of 3000 kW/km/GW.
Consideration has been given to using superconductors with a low critical temperature for these transmission lines since they can transport very high current densities with very low losses. This type of transmission line requires a substantial and complex cryogenic plant because of the need to maintain the low temperature, which is close to the temperature of liquid helium (4.2.degree.K). Substantial and therefore costly insulation is also required to limit cryogenic losses to an acceptable level. For this reason there are no such transmission lines in use at this time.
The introduction of superconductive ceramics with a high critical temperature, for example a temperature above that of liquid nitrogen, indicates the possibility of implementations that are favorable from both the economic and technical points of view.
The use of superconductors to limit short-circuit currents has also been investigated. This application exploits the property of the superconductive material of switching from a superconductive state with virtually zero resistance to a so-called normal state with a very high resistance beyond a critical value of the current. In this way it is possible to limit the value of a short-circuit current to a particular value, for example five times the nominal current, where without such limitation it could increase to at least 20 times the nominal current, for example. This limitation prevents the damage that can otherwise be done by the short-circuit current and enables a substantial saving in terms of the rating of the lines, the generators, the transformers and the switching plant.
A current limiter is usually regarded as an additional device to be connected in series into the line to be protected.
One object of the present invention is to provide a superconductive transmission line with two functions: transportation of current and current limitation, and which contributes the advantages and performance of each function.